The highest performance superalloy currently in production is one identified as PWA 1422 (DS 200+Hf); its density is 8.55 g/cm.sup.3 (grams per cubic centimeter). This alloy has columnar grains which are obtained by directional solidification giving high resistance to forces in a direction parallel to the grain boundaries.
Much work is currently in progress to devise new compositions suitable for making monocrystalline blades by directional solidification.
One of the best alloys of this type which has been proposed, is identified as PWA 1480 (or alloy 454), and its density is 8.7 g/cm.sup.3.
The creep resistance of this alloy, by comparison to the above-mentioned columnar grain alloy (PWA 1480), provides an improvement of 20.degree. C. to 50.degree. C. (depending on the temperature range under consideration).
This property stems from the composition of the alloy PWA 1422 and also from precipitation heat treatments applied to the parts made from the alloy after monocrystalline solidification, e.g. the treatments described in the commonly owned French patent application published under the U.S. Pat. No. 2,503,188 in which a .gamma.' phase is precipitated with an average crystal size of 5,000 .ANG.ngstroms.
However, for alloys that are intended to constitute the moving blades of turbomachines, only the specific creep characteristics are really important in practice, bearing in mind that the density of the alloy must be as low as possible to minimize centrifugal stress on the moving blades of turbomachines.
It is currently accepted that an increase of about 10% in density leads to a reduction in the usefulness of a disk on which the blades are mounted by a factor of 3.
Conversely, a reduction in density leads to a lighter blade-disk assembly, thereby improving turbomachine performance.
However, up to the present, the density of known low creep monocrystalline superalloys has been about 8.6 g/cm.sup.3.
One aim of the present invention is to provide an alloy composition which enables monocrystalline parts to be obtained having specific creep characteristics which are better than those of the alloy 454, but for which the density does not exceed 8.25 g/cm.sup.3, i.e. showing an improvement of about 5% over the said known alloy.
In general, improved resistance to creep when hot is obtained by massive addition of refractory elements such as Ta, W, and Mo or Re.
Thus the strongest monocrystalline alloy now being used industrially (alloy 454) contains 12% Ta and 4% W.
Similarly the alloy DS 200+Hf contains 12% W.
These refractory elements play an important part in reducing the rate of creep, and correspondingly in increasing useful operating life.
These elements have very low diffusion rates, even at high temperatures, thereby slowing down the coalescence rate of the .gamma.' phase, i.e. the hardening phase Ni.sup.3 (Al, Ti, . . . ) on which the alloy's resistance to creep when hot depends.
However, these refractory elements are very heavy, and while they do effectively increase the resistance to creep when hot, they also have the drawback of simultaneously increasing the density of the alloy.
It may be thought that the density of the alloy can be reduced by adding large quantities of light elements such as aluminum, but this leads to primary precipitation of the .gamma.' phase and the alloy thus does not have the required creep characteristics.